Problem: In chemistry class, the girl to boy ratio is $5$ to $5$. If there are $20$ boys, how many girls are there?
Solution: A ratio of $5$ girls to $5$ boys means that there is a group of $5$ girls for every group of $5$ boys. If there are $20$ boys, then there are $4$ groups of $5$ boys each. Because we know the ratio is $5:5$ , there must be $4$ groups of $5$ girls each. There is a total of $20$ girls in chemistry class.